The development of optical communication systems using silica-based optical fibers has stimulated great interest in light sources and photodetectors capable of operating within the wavelength region, generally between approximately 0.7 .mu.m and approximately 1.6 .mu.m, of interest to such systems. A photodetector is an essential component of such a system and as a result, much effort has been directed toward developing structures and materials for photodetectors.
The photodetectors presently contemplated for use with fiber-based optical communication systems are capable of high speed operation and fall into several general categories. First, there are p-i-n photodiodes. However, these suffer the drawback of not having any current gain. Second, avalanche photodiodes have been developed and these, of course, have current gain. However, the avalanche process introduces noise which is undesirable for many communication systems purposes. Third, phototransistors have been developed. However, these devices generally have low optical gain in the low incident power regime where optical communication systems operate and where high gain is most critically needed. Additionally, they generally have relatively slow speed because of minority carrier storage in the base region.
A high speed photodetector which does not require a bias voltage would be desirable for optical communication systems. The high speed photodetectors of the categories briefly described above usually require a bias voltage which is typically in the range between several volts and several hundred volts. For example, avalanche photodiodes generally require a bias voltage of at least 20 volts to enhance the avalanche process. Even p-i-n photodiodes are usually biased at several volts to obtain high speed response times. See for more details, for example, the article by H. Melchior on detectors for lightwave communication in Physics Today, pp. 32-39, November 1977.
Other photodetectors also have drawbacks associated with the presence of a bias voltage. For example, the bias voltage in p-n junction photodetectors increases the electric field and widens the depletion width and thus ensures that the optical absorption occurs in a region having a high electric field. While this eliminates the slow diffusion process of the carriers, it also gives rise to a d.c. dark current and hence contributes to shot noise, thereby degrading device characteristics. In general, the reverse bias leakage current depends exponentially on both the energy bandgap of the material and the ambient temperature. Consequently, problems associated with the magnitude of the leakage current can become especially severe for narrow bandgap materials. To some extent, the deleterious effects caused by these problems can be alleviated by placing the p-n junction in a higher bandgap material for a long wavelength detector having a very low dark current. See Electronics Letter, 16, pp. 893-895, 1980.